Hi. I have a question about power ratings for speakers: why do some speakers have a minimum power rating? For example, I have some old Mission bookshelf speakers that are rated for "25 - 125 Watts". These speakers can obviously be run at less than 25 watts (in fact, under usual operation they're probably running at 0.1 watt), so what's this rating for? I'm guessing that it's either the mechanical power rating for the woofer (because a 25 watt amp does in fact max out the woofer at low frequencies), or it's the range of amplifier power levels that Mission recommends for these speakers.

You can power them with a boom box amp if you want. What the manufacturer is stating for minimum power requirements is what they feel is the least amount of power that will properly drive those speakers. Anything less and you may not be getting adequate sound volume without distortion/clipping. In fact driving those speakers with a 25 watt amp at full volume would probably cause clipping in the amp and heard as distortion in the speakers. It would be better to over power rather than under power a speaker.

Play "Pictures at an Exhibition" and during the quiet passages, you're probably at less than .1 watt until all hell breaks loose. Then, you probably have instantaneous peaks of 25-50 watts. The minimum power is a suggestion so you don't compress the peaks (clipping) and cause tweeter damage and generally lousy sound. Watts are cheap, so why skimp?

some enclosure designs, like sealed "acoustic suspension", need a minimum level of power to overcome the resistance provided by the air trapped in the enclosure. Yes, they will make sound at lower levels, but not the acoustic sound reproduction they were designed for.

Its just there to tell you that anything less than that wattage may adversely affect the sound or damage the tweeters.

Its just there to tell you that anything less than that wattage may adversely affect the sound or damage the tweeters.

Less wattage isn't going to do anything to the tweeters. The third harmonic from a square wave is 9.5dB down from the fundamental, the fifth harmonic -14dB, and seventh harmonic -16.9dB; and clipping doesn't get you a perfect square wave.

Since the musical spectrum favors lower frequencies (the approximation used in the IEC power handling test is pink noise with a 6dB crest factor, second order high-pass at 40Hz, and second order low pass at 5 KHz) those clip first and can't get any louder when you continue to turn up the volume.

The high frequencies do get louder, and since speakers are designed to handle a musical power spectrum you're more likely to overload the tweeters.

If you don't act like a drunken teenager who continues to turn up the volume once it sounds distorted you're not going to have problems, although you may be unable to achieve a satisfactory listening volume without buying a more powerful amplifier or more sensitive speakers.

some enclosure designs, like sealed "acoustic suspension", need a minimum level of power to overcome the resistance provided by the air trapped in the enclosure. Yes, they will make sound at lower levels, but not the acoustic sound reproduction they were designed for.

There's no minimum power requirement, although small boxes with deep bass extension can't be efficient and may not produce acceptable loudness with a lower powered amplifier. "small" and "deep" are relative.

You can model the system as a pair of springs in parallel with no difference on SPL getting the stiffness from the box air spring or driver suspension. Smaller boxes cause more distortion due to differences in the spring force for inward and outward motion but that's a separate issue.

Less wattage isn't going to do anything to the tweeters. The third harmonic from a square wave is 9.5dB down from the fundamental, the fifth harmonic -14dB, and seventh harmonic -16.9dB; and clipping doesn't get you a perfect square wave.

Since the musical spectrum favors lower frequencies (the approximation used in the IEC power handling test is pink noise with a 6dB crest factor, second order high-pass at 40Hz, and second order low pass at 5 KHz) those clip first and can't get any louder when you continue to turn up the volume.

The high frequencies do get louder, and since speakers are designed to handle a musical power spectrum you're more likely to overload the tweeters.

If you don't act like a drunken teenager who continues to turn up the volume once it sounds distorted you're not going to have problems, although you may be unable to achieve a satisfactory listening volume without buying a more powerful amplifier or more sensitive speakers.

Thanks for clearing that up, I forgot to mention clipping.

What does IEC stand for? :confused:

Thanks for clearing that up, I forgot to mention clipping.

What does IEC stand for? :confused:

International Electrotechnical Commision. They're an international electrical/electronics standards organization.

IEC 268-5 covers the noise signal typically used in speaker power ratings. It's a reasonable upper bound for a musical power spectrum and commonly used to rate complete speakers and individual drivers. Tweeters get rated to X Watts with their manufacturer's specified high pass frequency and slope applied to the signal.

IEC 60320 covers the C13 "IEC plug" used on most equipment with detachable power cords and the C14 inlet receptacle.